The anaerobic organism, Bacteroides fragilis is exceptionally resistant to the toxic effects of oxygen. This resistance can be attributed to induction of an oxidative stress response (OSR) and this response will be studied to document its role in the pathogenesis of Bacteroides infections. It is expected that new mechanisms of free radical protection and novel antioxidant defense strategies will be uncovered. This idea is supported by the fact that B. fragilis has diverged early from other eubacteria and has independently evolved the ability to survive extended exposure to oxygen. Further, since this is an anaerobe it is likely to have stringent requirements for oxygen protection that have resulted in the development of unique highly protective antioxidants. The long-term goals of this research are to understand the basic physiological and genetic processes responsible for B. fragilis oxygen tolerance and how these contribute to virulence. Specific objectives for this proposal are: 1) Delineate novel mechanisms that regulate expression and function of OxyR. OxyR is a redox sensitive transcriptional regulator that governs expression of the OSR. This aim proposes to pursue analysis of unique mechanisms through which OxyR regulates genes expression. In addition we will test the hypothesis that the B. fragilis thioredoxin system is the predominant system for maintaining the cellular redox balance and as such it controls the redox state of oxidative stress responsive regulatory proteins such as OxyR. 2) Characterize the role of Fur homologs in control of the OSR. This aim will test the hypothesis that the three Fur homologs in B. fragilis are required for protection during oxidative stress due to their regulation of metal (Fe) homeostasis and other OSR genes. 3) Elucidate a role for the OSR in B. fragilis infections. This aim will address the hypothesis that the OSR is necessary for initiation of infection and for persistence in the host. Our approach is composed of two phases that will examine the effect of OSR mutations on survival in animal models that mimic the two different stages of infection. Understanding oxygen tolerance may lead to identification of new antibiotic targets effective for treatment of B. fragilis infections. For example, exclusive dependence on thioredoxins for redox control may be a weakness that can be exploited by drugs targeted against thioredoxin reductase. [unreadable] [unreadable]